4.x: Fix InvalidOp when enumerating the SystemClockChanged hashset

[akarnokd]

When the clock change is notified, any modification to the SystemClockChanged hashset from the callbacks results in an InvalidOperationException. The fix is to create a copy of the hashset contents and enumerate that.

Resolves #879

[clairernovotny]

Is there any tests we can add to verify this behavior?

[akarnokd]

Looks complicated to do it: get in a scheduler that unregisters itself upon a time change signal and also trigger a time change event. Maybe if we expose the relevant private fields or methods (such as OnSystemClockChanged()).

[clairernovotny]

We can make things internal if needed, then I think the InternalsVisibleTo should let the tests call it. Another approach could be a new internal-only method that sets the values as desired just for testing?